My friends study the history of religions and say that, once Zeus was known for being the head of Olympus, while Poseidon was considered the chief deity. One piece of evidence they give for this is the trident.

Have you asked your friends to elaborate? And perhaps provide you with sources for their claims? That would really help answer your question. Poseidon's main domain was the ocean, so it stands to reason that he was considered a very important deity - and perhaps the most important one - in cities with strong naval traditions (like, for example, Corinth). I'm afraid this is all I have to offer until we clarify the question a bit.
– yannis♦Jan 26 '16 at 11:01

Thank you. Being a bad students, they can only refer to the "lecturer said". Anyway, your opinion is more important, as I live in a small, poor and old country. I'll try to get that lecturer to explain me.
– HovoJan 26 '16 at 11:16

3 Answers
3

Zeus, for the most part, seems to be represented as the head of the gods in most cases. For example:

He was the only one not eaten by Kronos, and managed to save his siblings

He was ruler of Olympus, home of the gods

Many of the heroes and kings mentioned in mythology are his children. This would include Perseus, Tantalus, Minos, etc (see the Wikipedia page for Zeus for more). In fact, six of the twelve gods of Olympus are his children- Athena, Hephaestus, Apollo, Artemis, Ares, and Dionysus.

Being ruler of the sky pretty much cinches him as the ruler. In ancient Greece, the sky was always above you, huge, unreachable, and full of mysterious powers they didn't understand. It would make sense if their chief deity was the controller of all that.

BUT

Although Zeus is generally portrayed as king god, remember, there is no "set in stone" version of the myth. Since the Bronze Age, they were generally told orally from one person to another, each embellishing and carving it to their own liking. The version of the myths as most people today know them were described (and, importantly, written down) by authors such as Ovid, Homer,and Hesiod. While it is likely that their version of the myths was similar to everyone else's, there is no reason to believe that everything had to be exactly as we know it. A lot can change in a few thousand years! Sure, something like "who's chief of the gods" is a mighty big thing to change, but as you mentioned, it would actually make sense for Poseidon to be the chief deity, even if Zeus was technically king. Here's why:

The Greeks (and the Romans, too, for that matter) lived on the Mediterranean. A map of their world looked something like this:

Most of their world, if you notice, was ocean, or more specifically, Oceanus, the personification of the sea that supposedly surrounded the world.

For the most part, Greece was a bunch of islands with some annoying mountains here and there. The sea was a huge part of their lives, whether they wanted it to be or not.

In fact, the sea would probably be as mysterious as the sky to them. They wouldn't know about currents, or how storms worked, or about how come there were tides. There's only so much land-based weather could do the Greeks. They are not really in a region prone to tornadoes or anything like that. However, storms on the ocean were probably more or less normal- and much more powerful.

And who ruled over this massive, dangerous, all-powerful sea? Poseidon. As if that wasn't enough, he was also given the title of Earthshaker, attributing even more power and destruction to him.

Much like Zeus, many of his children were significant in mythology, including Bellerophon, Charybdis, and Proteus.

From all this, it would make sense that the ancient people favored Poseidon over Zeus.

Note that I didn't mention the trident. That's because during a battle the Cyclopes gave each of the three elder gods a gift. Poseidon got his trident, Hades got the helmet of darkness which can make him disappear, and Zeus was given the lightning bolt. So they each got something to show their power and Poseidon's trident had nothing to do with it.

This answer is fundamentally flawed. If Poseidon was once the "supreme deity", but was replaced by Zeus, then the Greeks would be sure to modify their myths to remove any mention of Poseidon being the chief deity. Instead of trying to extrapolate the answer to this question from Greek mythology, why don't you try to answer this question using historical accounts of greek religion?
– user62Mar 18 '16 at 18:04

I actually think the analysis in this answer is valid, especially as the writer makes the point about the evolution of these myths. But for facts, as opposed to analysis, you should really be looking to @solsdottir's answer. NOTE: Linear B is an earlier form of Ancient Greek, so the references she mentions likely pre-date the Classical authors we're familiar with.
– DukeZhouOct 6 '16 at 21:02

Poseidon may well have been a chief god in some particular places. This would have been in the Mycenaean period, when Poseidon had important cults at Pylos and Thebes. Also, on the Linear B tablets his name is more common than Zeus', along with a feminine variant that was probably his spouse. Other tablets record offerings to Poseidon and the Two Queens (assumed to be Demeter and Persephone).

Adding to @solsdottir's excellent answer, these things were not as fixed as they now seem, millennia later. The formation of these mythologies was certainly a process, beginning in the oral tradition and migrating to text. A very strong theme across the body of Classical mythology is transformation, and not just in the literal sense of Ovid's famous work, but in terms of status and position of various deities and heroes.
– DukeZhouOct 6 '16 at 20:27

No, he was not. Zeus was the only god not to be eaten by Kronos, and therefore had to defeat him alone, and was crowned king of the gods for it. As for Poseidon having the trident, Hades has his helm of darkness, and Zeus has his lightning bolts.